1. Field of the Invention
This invention relates to the sintering of metal interlayers within organic polymeric films.
2. Background
There exist in the art many different chemical or physical methods by which a zero-valent metal can be added to a polymeric film. Among these, some are capable of producing a thin continuous coating at a surface of the polymer, for example, meal vapor deposition or electroless plating, as disclosed by R. W. Berry, P. M. Hall, and M. T. Harris, "Thin Film Technology", D. van Nostrand Co., Inc.; Princeton, N.J. 1968, pp. 1-17. The electrical continuity and/or optical reflectivity of such a coating forms the basis for a variety of technological applications. Other processes, such as silver halide photography (A. Roff and E. Weyde, "Photographic Silver Halide Diffusion Processes", The Focal Press, London and New York, 1972, pp. 13-31) and certain forms of chemical (R. C. Haushalter and L. J. Krause, Thin Solid Films, 102, 161 (1983)) or electrochemical (J. A. Bruce, J. Murahashi, and M. S. Wrighton, J. Phys. Chem., 86, 1552 (1982)) deposition, produce metal particles embedded within the polymer film. However, these particles are generally dispersed to such a degree that they lack the characteristic electrical or optical properties of a continuous metal layer.
Certain organic or organometallic polymers have been described in the literature, for example, J. A. Bruce and M. S. Wrighton, J. Am. Chem. Soc., 104, 74 (1982), and publications cited therein, as "electrochemically active". When coated as a film on the surface of an electrode, these polymers can accept and/or donate electrons to the electrode at a potential dictated by the redox potential of the polymer. This redox process may involve not only the surface of the polymer in contact with the electrode, but all of the redox-active groups throughout the sample. This behavior requires that both electrons and counterions have some finite mobility within the polymer.
A report by P. G. Pickup, K. M. Kuo, and R. J. Murray, J. Electrochem. Soc., 130, 2205 (1983), describes their study of electrodeposition of metals (Cu, Ag, Co and Ni) from solution onto electrodes coated with the electrochemically active polymer poly-[Ru(bpy).sub.2 (vpy).sub.2 ].sup.n+. At potentials sufficiently negative to reduce the polymer to Ru(I) or Ru(O) these authors report that the metal ions, for example, Cu.sup.+n, Ag.sup.+, are reduced by the polymer to form particles or films on the surface of the polymer facing the solution. They further consider the general aspects of such a process and speculate about alternative possible results from those observed. They conclude that the locus of metal deposition depends on the relative rates of the steps: (1) diffusion of metal ions through the polymer to the Pt electrode; (2) diffusion of electrons through the polymer from the Pt electrode; and (3) reduction of the metal ions by the reduced polymer. In particular, they conclude that in order to obtain metal deposition within the polymer, it would be necessary that steps 1 and 2 be equally fast and step 3 must be faster.
Haushalter and Krause, supra, disclose the chemical reduction of organic polymers, especially polyimides, by treatment with certain strongly-reducing main-group metal cluster compounds, Zintl ions. This process was employed for two different kinds of metallization processes. First, main group metals, derived from oxidation of the Zintl ions, were deposited on the surface of the polymer. Secondly, the reduced form of the polymer was reacted with metal salts from solution to generate zero-valent metal particles by a process which is formally equivalent to the electrochemical depositions of Bruce et al., and Pickup et al., supra. The Zintl ions were obtained either by extraction, for example, with ethylenediamine, of a Zintl phase (an alloy of a polyatomic main group element, for example, germanium, tin, lead, arsenic or antimony, with an alkali or alkaline earth metal) or by electrolysis of a main group electrode. Metallization of a polyimide of 4,4'-diaminodiphenyl ether, also referred to herein as 4,4'-oxydianiline, and pyromellitic anhydride, also referred to herein as pyromellitic dianhydride, is disclosed.
In U.S. Pat. No. 4,512,855 now U.S. Pat. No. 4,512,855, S. Mazur discloses a process which is capable of producing, in a single step, a thin layer of metal completely embedded within a polymeric film, that is, an interlayer. By means of this process, it is possible to control both the thickness of the interlayer(s) and its position(s) within the polymeric film. Most notably, such an interlayer(s) may possess sufficient continuity and planarity to exhibit electrical and optical characteristics hitherto available only with surface layers. By "interlayer" in meant a discrete metal laminar region embedded within, and parallel to, the polymeric film, the laminar region being thinner in the transverse direction than said film and the density of the metal within said region being greater than the density of metal on either side of the region.
Sintering of metal particles is employed in the art for a broad range of commercially important applications with the object of increasing the mechanical inegrity and/or electrical conductivity of a "green formed" object. The general utility of sintering resides in the fact that it occurs at temperatures much below the melting point of the metal; therefore, the original geometric shape of the object is maintained. The green formed object may be composed purely of metal particles pressed together, for example, by compression molding, or it may also contain an organic material, such as a polymer, acting as a binder. In the latter instance (see, for example, U.S. Pat. Nos. 4,197,118 and 4,283,360) the polymer is a transient component of the system and is either removed or decomposed during the sintering process. It is not a functional component of the final sinered part.
There are many examples of the use of a laser to selectively alter the optical or electrical properties of thin metal films. In most instances these metal films are supported on the surface of a substrate (for example, glass, ceramic, organic polymer). Thus, ablation of thin metal surface layers has been used to create conductive circuit patterns (Paek and Kestenbaum, J. Appl. Phys., 44, 2260 (1973)). Also, such techniques have been developed for application in the area of optical information storage, where digital information is encoded as microscopic dots, distinguished from their background by virtue of a difference in reflectivity. This information may be written onto or into a suitable medium by means of a laser. As described by Jipson and Ahn, Solid State Technology, p. 141, January, 1984, and Drexler, J. Vac. Sci. Technol., 18, 87 (1981), a number of different principles have been exploited to obtain the necessary laser sensitivity:
1. Ablation: A thin layer on top or embedded within a substrate is ablatively removed by the laser. PA0 2. Bilayer Alloying: A eutectic alloy is created at the interface between two components of the active layer. PA0 3. Smoothing of a Textured Surface: The surface of the active layer is prepared in a roughened antireflective form. Heat from the laser melts the surface and smooths it to increase the reflectivity. (Craighead and Howard, Appl. Phys. Lett., 38, 1981). PA0 4. Island Formation: Where an extremely thin fold film is heated by the laser, it coalesces into isolated islands, resulting in greatly reduced reflectivity. PA0 5. Vesicle Formation: Heat from the laser causes formation of a vesicle or bubble between the active layer and the substrate. Distortion of the surface reduces its reflectivity. PA0 6. Phase Change: Laser heating of a glassy, optically transmissive material causes it to crystallize. The crystalline form scatters light and thereby prevents reflection from a secondary layer beneath the active layer. PA0 7. Distortion of the Substrate: A reflective metal layer absorbs the light and causes local heating of the substrate. Melting of the substrate results in distortion of the reflective surface with consequent decrease in reflectivity.
Principle 3 above employs a sintering process to smooth an optically rough surface. The use of sintering to enhance the reflectivity of an already optically flat surface is not disclosed in the art.
It is an object of this invention to provide a method for systematically altering the electrical conductivity and optical characteristics (reflectivity and transmissivity) of the aforesaid interlayer disclosed by S. Mazur (U.S. Pat. No. 4,512,855) so as to provide an interlayer having properties ranging from those of the bulk metal to those of the polymer matrix, without degrading or otherwise adversely affecting the polymer. Other objects will become apparent hereinafter.